In this paper we consider the introduction of a body force, in the incompressible limit, into the lattice
Boltzmann model. A number of methods are considered and their suitability to our objectives determined.
When there is no density variation across the fluid, gravity can be introduced in the form of an altered pressure
gradient. This method correctly satisfies the Navier-Stokes equation; however, if there is a non-negligible
density variation present ~produced by the body force or otherwise! this method becomes less accurate as the
density variation increases and the constant density approximation becomes less valid. Three other methods are
also considered for application when there is a non-negligible density variation. The equations of motion
satisfied by these models are found up to second order in the Knudsen number and it is seen that only one of
these methods satisfies the true Navier-Stokes equation. Numerical simulations are performed to compare the
different models and to assess the range of application of each.